Chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings

ABSTRACT

A coated steel substrate for packaging applications, the substrate containing (i) a conventional non-passivated electrolytic, optionally flowmelted, tinplate, or (ii) a cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate, wherein one or both sides of the substrate is coated with a chromium metal—chromium oxide coating layer produced in a single process step by using a trivalent chromium electroplating process. A process for obtaining the coated steel substrate.

This invention relates to chromium-chromium oxide (Cr—CrOx) coatings applied to steel substrates for packaging applications and to a method for producing said coatings.

Tin mill products include tinplate, Electrolytic Chromium Coated Steel (ECCS, also referred to as tin free steel or TFS), and blackplate, the uncoated steel. Packaging steels are normally provided as tinplate, or as ECCS onto which an organic coating can be applied. In case of tinplate this organic coating is usually a lacquer, whereas in case of ECCS increasingly polymer coatings such as PET or PP are used, such as in the case of Protect®.

Tinplate is characterised by its excellent corrosion resistance and weldability. Tinplate is supplied within a range of coating weights, normally between 1.0 and 11.2 g/m², which are usually applied by electrolytic deposition. At present, most tinplate is post-treated with fluids containing hexavalent chromium, Cr(VI), using a dip or electrolytically assisted application process. Aim of this post-treatment is to passivate the tin surface to stop or reduce the growth of tin oxides, because too thick oxide layers can eventually lead to problems with respect to adhesion of organic coatings, like lacquers. It is important that the passivation treatment should not only suppress or eliminate tin oxide growth, but should also be able to retain or improve organic coating adhesion levels. The passivated outer surface of tinplate is extremely thin (less than 1 micron thick) and consists of a mixture of tin and chromium oxides.

ECCS consists of a blackplate product which has been coated with a metallic chromium layer overlaid with a film of chromium oxide, both applied by electrolytic deposition. ECCS excels in adhesion to organic coatings and retention of coating integrity at temperatures exceeding the melting point of tin (232° C.). In those cases tinplated material cannot be used. This is important for producing polymer coated packaging steel because during the thermoplastic coating application process the steel substrate may be heated to temperatures exceeding 232° C., with the actual maximum temperature values used being dependent on the type of thermoplastic coating applied. This heat cycle is required to enable initial heat sealing/bonding of the thermoplastic to the substrate (pre-heat treatment) and is often followed by a post-heat treatment to modify the properties of the polymer. The chromium oxide layer is believed to be responsible for the excellent adhesion properties of thermoplastic coatings such as polypropylene (PP) or polyester terephthalate (PET) to ECCS. ECCS can also be supplied within a range of coating weights for both the Cr and CrOx coating, typically ranging between 20-110 and 2-20 mg/m² respectively. ECCS can be delivered with equal coating specification for both sides of the steel strip, or with different coating weights per side, the latter being referred to as differentially coated strip. The production of ECCS currently involves the use of solutions on the basis of chromium in its hexavalent state, also known as hexavalent chromium or Cr(VI).

Hexavalent chromium is nowadays considered a hazardous substance that is potentially harmful to the environment and constitutes a risk in terms of worker safety. There is therefore an incentive to develop alternative metal coatings that are able to replace conventional tinplate and ECCS, without the need to resort to the use of hexavalent chromium during manufacturing.

It is an objective of the invention to provide an alternative to the use of hexavalent chromium for the passivation of tinplate.

It is an objective of the invention to provide an alternative to conventional tinplate to improve the product properties e.g. in terms of corrosion performance and sulphur staining resistance.

It is also an objective of the invention to provide an alternative substrate to tinplate and ECCS which provides excellent dry adhesion to organic coatings in combination with corrosion protection that does not rely on the use of hexavalent chromium during manufacturing.

One or more of these objects are reached by providing a packaging steel substrate containing:

-   1. a conventional non-passivated electrolytic, optionally     flowmelted, tinplate (i.e. ETP), or -   2. a cold-rolled and recovery annealed electrolytic, optionally     flowmelted tinplate     wherein one or both sides of the substrate is coated with a chromium     metal—chromium oxide (Cr—CrOx) coating layer produced in a single     plating step by using a trivalent chromium electroplating process.

The packaging steel substrate is preferably provided in the form of a strip.

For the production of ECCS generally three types of chromium plating processes are in use throughout the world. The three processes are “one step vertical process” (V-1), “two step vertical process” (V-2), and the “one step horizontal high current density process” (HCD) and based on Cr(VI) electrolytes. The specifications of ECCS are standardized under Euronorm EN 10202:2001. The two-step vertical process uses a sulphuric acid free Cr(VI) electrolyte for applying the chrome oxide layer in the second step. Sulphuric acid is needed for a good efficiency in applying chrome metal and is therefore always used for the chrome metal plating step in these processes. The “one step vertical” and the “one step horizontal high current density (HCD) process” always have sulphate in the oxide layer because the chromium metal and chromium oxide are produced simultaneously in the same electrolyte (Boelen, thesis TU Delft 2009, page 8-9, ISBN 978-90-805661-5-6). In all cases the ECCS consists of a chromium oxide layer on top of the chromium metal.

In the process according to the invention a coating layer comprising chromium metal and chromium oxide is deposited, and not by first depositing a chromium metal layer, and then providing a chromium oxide layer on top as a conversion layer. The Cr—CrOx layer should consist of a mixture of Cr-oxide and Cr-metal and the Cr-oxide should not be present as a distinct layer on the outermost surface, but mixed through the whole layer Cr—CrOx. Of course there may be more than one of these single plating steps one after the other if, for instance, a thicker coating layer comprising chromium metal and chromium oxide layer is to be deposited. The phrase single plating step is therefore not limited to mean that only one of these single plating steps is used.

The packaging steel substrate is usually provided in the form of a strip of low carbon (LC), extra low carbon (ELC) or ultra low carbon (ULC) with a carbon content, expressed as weight percent, of between 0.05 and 0.15 (LC), between 0.02 and 0.05 (ELC) or below 0.02 (ULC) respectively. Alloying elements like manganese, aluminium, nitrogen, but sometimes also elements like boron, are added to improve the mechanical properties (see also e.g. EN 10 202, 10 205 and 10 239). In an embodiment of the invention the substrate consists of an interstitial-free low, extra-low or ultra-low carbon steel, such as a titanium stabilised, niobium stabilised or titanium-niobium stabilised interstitial-free steel.

It was found that a chromium metal—chromium oxide (Cr—CrOx) coating produced from a trivalent chromium based electroplating process provides excellent adhesion to organic coatings. In this aspect, the chromium metal—chromium oxide (Cr—CrOx) coating produced from a trivalent chromium electrodeposition process has very similar adhesion properties compared to conventional ECCS produced via a hexavalent chromium electrodeposition process. By increasing the thickness of the Cr—CrOx coating layer the porosity of the coating is reduced and its corrosion resistance properties improve.

The Cr—CrOx coating can be applied onto conventional, non-passivated, electrolytic, and optionally flowmelted, tinplate (ETP, Electrolytic Tinplate). The Cr—CrOx layer ensures that the growth of tin oxides is suppressed, i.e. it has a passivation function. With increasing Cr—CrOx thickness it was unexpectedly found that the wet adhesion performance, i.e. the organic coating adhesion after sterilisation, outperforms conventional hexavalent chromium passivated tinplate. In addition, the resistance to so-called sulphur staining, i.e. the brown discolouration of tinplate due to contact with sulphur containing fill-goods, can be fully suppressed by applying a sufficiently thick Cr—CrOx coating. The material according to the invention is therefore very suitable for replacement of hexavalent chromium passivated tinplate, optionally exceeding the technical performance limits of standard tinplate. From a process point of view, the fact that the Cr—CrOx coating layer is applied in a single process step means that two process steps are combined, which is beneficial in terms of process economy and in terms of environmental impact.

Alternatively the Cr—CrOx coating can also be applied directly onto the blackplate packaging steel substrate, without prior application of a tin coating, i.e. directly applied onto the bare steel surface. According to Merriam Webster blackplate is defined as sheet steel that has not yet been made into tin plate by being coated with tin or that is used uncoated where the protection afforded by tin is unnecessary. It was found that the dry adhesion levels to organic coatings for both thermoset lacquers and thermoplastic coatings, of this material can approach those normally associated with the use of ECCS. The material according to the invention can be used to directly replace ECCS for applications that require a moderate corrosion resistance.

The big advantage, both in terms of environmental impact and health and safety is the fact that with this invention the use of hexavalent chromium chemistry is prevented, while it is possible to retain the product performance properties normally attributed to ECCS and tinplate.

In an embodiment the Cr—CrOx coating layer applied onto non-passivated tinplate contains at least 20 mg Cr/m², to create a tin oxide passivating effect. This thickness is adequate for many purposes.

In an embodiment the Cr—CrOx coating layer applied onto non-passivated tinplate contains at least 40 mg Cr/m², preferably at least 60 Cr/m², to create a tin oxide passivating effect and to prevent or eliminate sulphur staining. To prevent or eliminate sulphur staining, a layer of 20 mg Cr/m² was found to be too thin. Starting at thicknesses of about 40 mg Cr/m² the sulphur staining is already much reduced, whereas at a layer thickness of at least about 60 mg Cr/m² sulphur staining is practically eliminated.

A suitable maximum thickness was found to be 140 mg Cr/m². Preferably the Cr—CrOx coating layer applied onto non-passivated tinplate contains at least 20 to 140 mg Cr/m², more preferably at least 40 and/or at most 90 mg Cr/m², and most preferably at least 60 and/or at most 80 mg Cr/m².

These embodiments aim to replace hexavalent chromium passivated tinplate. The major advantage besides the elimination of hexavalent chromium from manufacturing is the potential to create a product with superior sulphur staining resistance and improved corrosion resistance.

It was found that the colour of the material changes with increasing Cr—CrOx layer thickness, with the product becoming darker (i.e. lower L-value) with increasing coating thickness. As the optical properties of packaging steels are very important to create an attractive aesthetic appearance of metal containers, like aerosol cans, this could be considered a drawback of the invention for specific applications. However, one way to circumvent these issues would be to use a differential coating, e.g. to use a low Cr—CrOx coating weight on one side of the material, while applying a thicker Cr—CrOx coating weight at the other side. The surface containing a thicker Cr—CrOx coating weight should be used for the inside of the container, to make use of the benefits of the improved corrosion resistance properties. In that case, the surface with the lower Cr—CrOx coating weight is on the outside of the container, for which the corrosion resistance requirements are usually less severe, ensuring optimal optical properties.

In an embodiment the Cr—CrOx coating layer applied onto blackplate is at least 20 mg Cr/m², to create a material that approaches the functionality of ECCS (e.g. excellent adhesion to organic coatings in combination with a moderate corrosion resistance). Preferably the Cr—CrOx coating layer applied onto blackplate is at least 40 and more preferably at least 60 mg Cr/m². A suitable maximum thickness was found to be 140 mg Cr/m². Preferably the Cr—CrOx coating layer applied onto blackplate contains at least 20 to 140 mg Cr/m², more preferably at least 40 mg Cr/m2, and most preferably at least 60 mg Cr/m². In an embodiment a suitable maximum is 110 mg Cr/m².

The Cr—CrOx coated blackplate aims to replace ECCS. The major advantage besides the elimination of hexavalent chromium from manufacturing is the potential to create a product for applications for which the superior corrosion resistance properties of tinplate are not required. From a process point of view, the fact that the Cr—CrOx coating layer is applied in a single process step means that two process steps are combined, which is beneficial in terms of process economy and in terms of environmental impact.

The Cr—CrOx coating can also be applied to a cold-rolled and recovery annealed blackplate, or to a cold-rolled and recovery annealed electrolytic, and optionally flowmelted, tinplate. These substrates have a recovery annealed substrate, rather than the recystallised single reduced ETP or blackplate or the double reduced blackplate. The difference in microstructure of the substrate was not found to materially affect the Cr—CrOx coating.

It was found that the material according to the invention can be used in combination with thermoplastic coatings, but also for applications where traditionally ECCS is used in combination with lacquers (i.e. for bakeware such as baking tins, or products with moderate corrosion resistance requirements) or as a substitute for conventional tinplate for applications where requirements in terms of corrosion resistance are moderate.

In an embodiment the coated substrate is further provided with an organic coating, consisting of either a thermoset organic coating, or a thermoplastic single layer polymer coating, or a thermoplastic multi-layer polymer coating. The Cr—CrOx layer provides excellent adhesion to the organic coating similar to that achieved by using conventional ECCS.

In a preferred embodiment the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising the use of thermoplastic resins such as polyesters or polyolefins, but can also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers. For clarification:

Polyester is a polymer composed of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include therephthalic acid, isophthalic acid, naphthalene dicarboxylic acid and cyclohexane dicarboxylic acid. Examples of suitable glycols include ethylene glycol, propane diol, butane diol, hexane diol, cyclohexane diol, cyclohexane dimethanol, neopentyl glycol etc. More than two kinds of dicarboxylic acid or glycol may be used together.

Polyolefins include for example polymers or copolymers of ethylene, propylene, 1-butene, 1-pentene, 1-hexene or 1-octene.

Acrylic resins include for example polymers or copolymers of acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester or acrylamide.

Polyamide resins include for example so-called Nylon 6, Nylon 66, Nylon 46, Nylon 610 and Nylon 11.

Polyvinyl chloride includes homopolymers and copolymers, for example with ethylene or vinyl acetate.

Fluorocarbon resins include for example tetrafluorinated polyethylene, trifluorinated monochlorinated polyethylene, hexafluorinated ethylene-propylene resin, polyvinyl fluoride and polyvinylidene fluoride.

Functionalised polymers for instance by maleic anhydride grafting, include for example modified polyethylenes, modified polypropylenes, modified ethylene acrylate copolymers and modified ethylene vinyl acetates.

Mixtures of two or more resins can be used. Further, the resin may be mixed with anti-oxidant, heat stabiliser, UV absorbent, plasticiser, pigment, nucleating agent, antistatic agent, release agent, anti-blocking agent, etc. The use of such thermoplastic polymer coating systems have shown to provide excellent performance in can-making and use of the can, such as shelf-life.

According to a second aspect, the invention is embodied in a process for producing a coated steel substrate for packaging applications, the process comprising the electro-deposition of a chromium metal—chromium oxide coating on the substrate with the electrolytic deposition on said substrate of said chromium metal—chromium oxide coating occurring in a single plating step from a plating solution comprising a trivalent chromium compound, an optional chelating agent, an optional conductivity enhancing salt, an optional depolarizer, an optional surfactant and to which an acid or base can be added to adjust the pH.

In an embodiment the electro-deposition of the Cr—CrOx coating is achieved by using an electrolyte in which the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation and the depolarizer comprises a bromide containing salt.

In an embodiment the cationic species in the chelating agent, the conductivity enhancing salt and the depolarizer is potassium. The benefit of using potassium is that its presence in the electrolyte greatly enhances the electrical conductivity of the solution, more than any other alkali metal cation, thus delivering a maximum contribution to lowering of the cell voltage required to drive the electro-deposition process.

In an embodiment of the invention the composition of the electrolyte used for the Cr—CrOx deposition was: 120 g/l basic chromium sulphate, 250 g/l potassium chloride, 15 g/l potassium bromide and 51.2 g/l potassium formate.

The pH was adjusted to values between 2.3 and 2.8 measured at 25° C. by the addition of sulphuric acid.

According to the invention the chromium containing coating is preferably deposited from the trivalent chromium based electrolyte at a bath temperature of between 40 and 70° C., preferably of at least 45° C. and/or at most 60° C.

Surprisingly, it was found that it is possible to electro-deposit a chromium Metal—chromium oxide coating layer from this electrolyte in a single process step. From prior art, it follows that addition of a buffering agent to the electrolyte, like e.g. boric acid, is strictly required to enable the electro-deposition of chromium metal to take place. In addition, it has been reported that it is not possible to deposit chromium metal and chromium oxide from the same electrolyte, due to this buffering effect (with a buffering agent being required for the electro-deposition of the chromium metal but excluding the formation of chromium oxides and vice versa). However, it was found that no such addition of a buffering agent was required to deposit chromium metal, provided that a sufficiently high cathodic current density is being applied.

XPS depth profiles were measured and the peaks that are measured are Fe2p, Cr2p, O1s, Sn3d, Cis. It was observed that the Cr-layer consists of a mixture of Cr-oxide and Cr-metal and that the Cr-oxide is not present as a distinct layer on the outermost surface, but is mixed through the whole layer. This is also indicated by the O-peak that is present in the whole Cr-layer. In all cases the Cr—CrOx layer has a shiny metallic appearance.

It is believed that a certain threshold value for the current density must be exceeded for the electro-deposition of chromium metal to occur, which is closely linked to pH at the strip surface reaching certain values as a result of the evolution of hydrogen gas and the equilibration of various (chelated) poly chromium hydroxide complexes. It was found that after crossing this threshold value for the current density that the electro-deposition of the chromium metal—chromium oxide coating layer increases virtually linearly with increasing current density, as observed with conventional electro-deposition of metals, following Faraday's law. The actual value for the threshold current density seems to be closely linked to the mass transfer conditions at the strip surface: it was observed that this threshold value increases with increasing mass transfer rates. This phenomenon can be explained by changes in pH values at the strip surface: at increasing mass transfer rates the supply of hydronium ions to the strip surface is increased, necessitating an increase in cathodic current density to maintain a specific pH level (obviously higher than the bulk pH) at the strip surface under steady-state process conditions. The validity of this hypothesis is supported by results obtained from experiments in which the pH of the bulk electrolyte was varied between a value of 2.5 and 2.8: the threshold value for the current density decreases with increasing pH value.

Concerning the electro-deposition process of Cr—CrOx coatings from trivalent chromium based electrolytes, it is important to prevent/minimise the oxidation of trivalent chromium to its hexavalent state at the anode and a suitable anode or anode material must be selected. By using a hydrogen gas diffusion anode as described below, the formation of Cr(IV) can be prevented.

In an embodiment of the invention the formation of Cr(IV) can be prevented by using one, more or only hydrogen gas diffusion anodes at which hydrogen gas (H₂(g)) is oxidised. H⁺ (protons) in an aqueous solution bind to one or more water molecules, e.g. as hydronium ions (H₃O⁺). The oxidation of H₂(g) to H⁺ (aq) prevents the occurrence of undesirable oxidation reactions, such as the formation of Cr(IV), which occur at a higher anodic overpotential when using an anode at which water (H₂O) is oxidised to oxygen (O₂(g)).

The reaction H₂(g)→2H⁺(aq)+2e⁻occurs at an anode potential of 0.00 V (SHE). The reaction 2H₂O→4H⁺(aq)+O₂(g)+4e⁻ occurs at an anode potential of 1.23 V (SHE). When an anode at which water is oxidised to oxygen is used, then reactions are possible which would not have been possible when using an anode at which hydrogen gas is oxidised.

One of such undesirable oxidation reactions is the oxidation of Cr(III) to Cr(VI) and this oxidation reaction can be completely excluded by using a hydrogen gas diffusion anode (GDA) at which H₂(g) is oxidised to H⁺.

In an embodiment of the method H₂(g) is oxidised at the gas diffusion anode to H⁺(aq) with a current efficiency of at least 99%, preferably of 100%. The higher the current efficiency, the smaller the likelihood of undesirable side reactions. It is therefore preferable that the current efficiency is at least 99%, and preferably 100%. Based on thermodynamic and kinetic considerations it can be argued that using a hydrogen gas diffusion anode completely eliminates the risk of Cr(III) oxidation as the anode operating potential is much too low for Cr(III) oxidation to occur.

Thermodynamically, under standard conditions (i.e. a temperature of 25° C. and a pressure of 1 atm) an electrode potential of >0 V is already sufficient for oxidising H₂(g) to H⁺(aq), whereas an electrode potential of >1.23 V is required for oxidising H₂O to O₂(g). Cr(III) can only be oxidised to Cr(VI) when the electrode potential is >1.35 V.

The electrode potential is measured against the standard hydrogen electrode. The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.44±0.02 V at 25° C., but to form a basis for comparison with all other electrode reactions, hydrogen's standard electrode potential (E⁰) is declared to be zero at all temperatures. Potentials of any other electrodes are compared with that of the standard hydrogen electrode at the same temperature.

The prevailing equilibrium (zero current) potential can be calculated from the Nernst equation by filling in the appropriate temperature, pressure and activities of the electro-active species. The anode operating (non-zero current) potential needed to generate a specific anodic current is determined by the activation overpotential (i.e. the potential difference required for driving the electrode reaction) and the concentration overpotential (i.e. the potential difference required to compensate for concentration gradients of electro-active species at the electrode).

Due to the low anode overpotential required for the oxidation of H₂(g) to H⁺(aq), the anode operating potential will always stay far below the value at which Cr(III) oxidation can take place (see FIG. 4 where the current is plotted against the anode potential in SHE). Firstly this results in a lower energy consumption of the electrodeposition process. Secondly, at an anode potential below about 1.35 V oxidation of Cr(III) to Cr(VI) is not possible (indicated with the crossed through arrow).

In an embodiment no depolariser is added to the electrolyte. When a hydrogen gas diffusion anode is used then the addition of a depolariser to the electrolyte is no longer needed.

The use of a hydrogen gas diffusion anode has the added advantage that the use of a chloride containing electrolyte becomes possible without the risk of chlorine formation. This chlorine gas is potentially harmful to the environment and to the workers and is therefore undesirable. This means that in the case of a Cr(III) electrolyte the electrolyte could be partly or entirely based on chlorides. The advantage of using a chloride based electrolyte is that the conductivity of the electrolyte is much higher compared to a sulphate only based electrolyte, which leads to a lower cell voltage that is required to run the electrodeposition, which results in a lower energy consumption.

The oxidation reaction of dissolved hydrogen on an active electrocatalyst surface is a very fast process. As the solubility of hydrogen in a liquid electrolyte is often low, this oxidation reaction can easily become controlled by mass transfer limitations. Porous electrodes have been specifically designed to overcome mass transfer limitations. A hydrogen gas diffusion anode is a porous anode containing a three-phase interface of hydrogen gas, the electrolyte fluid and a solid electrocatalyst (e.g. platinum) that has been applied to the electrically conducting porous matrix (e.g. porous carbon or a porous metal foam). The main advantage of using such a porous electrode is that it provides a very large internal surface area for reaction contained in a small volume combined with a greatly reduced diffusion path length from the gas-liquid interface to the reactive sites. Through this design the mass transfer rate of hydrogen is greatly enhanced, while the true local current density is reduced at a given overall electrode current density, resulting in a lower electrode potential.

A gas diffusion anode assembly to be used in the proposed electrodeposition method, typically comprises the use of the following functional components (see FIG. 5): a gas feeding chamber 1, a current collector 2 and a gas diffusion anode, which consists of an hydrophobic porous gas diffusion transport layer 3 combined with an hydrophilic reaction layer 4 (see FIG. 5). The latter is made up of a network of micropores that are (partly) drowned with liquid electrolyte.

Optionally, the reaction layer is provided with a proton exchange membrane on the outside 5, like a Nafion® membrane, to prevent the diffusion of chemical species (like anions or large neutral molecules) present in the bulk liquid electrolyte inside the gas diffusion anode, as these compounds can potentially poison the electrocatalyst sites, causing degradation in electrocatalytic activity.

The main function of the gas feeding chamber is to supply hydrogen gas evenly to the hydrophobic backside of the hydrogen gas diffusion anode. The gas feeding chamber needs two connections: one to feed hydrogen gas and one to enable purging of a small amount of hydrogen gas to prevent the build-up of gas phase contaminations potentially present in trace amounts in the hydrogen gas supplied. The gas feeding chamber often contains a channel type structure to ensure that hydrogen gas is distributed evenly over the hydrophobic backside.

The electrical current collector 2 is (usually) attached to the hydrophobic backside 3 of the hydrogen gas diffusion anode to enable the transport of the electrical current generated inside the anode to a rectifier (not shown in FIG. 5). This current collector plate must be designed in such a way to enable the hydrogen gas to contact the backside of the hydrogen gas diffusion anode so it can be transported to the reactive side inside the gas diffusion anode. Usually this is accomplished by using an electrically conductive plate with a large number of holes, a mesh or an expanded metal sheet made from e.g. titanium.

The functionality of gas feeding channels and electrical current collector can also be combined into a single component, which is then pressed against the hydrophobic back-side of the gas diffusion anode.

Once the hydrogen gas diffuses through the hydrophobic backside of the hydrogen gas diffusion anode it comes into contact with the electrolyte, which is present in the hydrophilic part of the anode, i.e. the reaction layer (see FIG. 5, right hand side). At the gas-liquid interface (between 3 and 4) the hydrogen gas dissolves into the electrolyte and is transported by diffusion to the electrocatalytic active sites of the hydrogen gas diffusion anode. Usually platinum is used as electrocatalyst, but also other materials like platinum-ruthenium or platinum-molybdenum alloys can be used. At the electrocatalytic sites the dissolved hydrogen is oxidised: the electrons that are generated are transported through the conductive matrix of the gas diffusion anode (usually a carbon matrix) to the current collector 2, while the hydronium ions (H⁺) diffuse through the proton exchange membrane into the electrolyte.

In an embodiment the coated substrate is further provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a lacquering step, or a thermoplastic single layer, or a thermoplastic multi-layer polymer by a film lamination step or a direct extrusion step.

In an embodiment the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising the use of thermoplastic resins such as polyesters or polyolefins, but can also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers; and/or copolymers thereof; and/or blends thereof.

Preferably, and particularly in the case of tinplate, the substrate is cleaned prior to Cr—CrOx electrodeposition by dipping the substrate in a sodium carbonate solution containing between 1 to 50 g/l of Na₂CO₃ at a temperature of between 35 and 65° C., and wherein the cathodic current density of between 0.5 and 2 A/dm² is applied for a period of between 0.5 and 5 seconds.

Preferably the sodium carbonate solution containing at least 2 and/or at most 5 g/l I of Na₂CO₃.

The invention is now further explained by means of the following, non-limiting examples and figures.

EXAMPLE 1

Sheets of conventional, non-passivated, flow melted tinplate (common steel grade and temper), with a tin coating weight of 2.8 g Sn/m² on both sides, were first given an electrolytic pre-treatment to minimise the tin oxide layer thickness. This was done by dipping the sheets into a sodium carbonate solution (3.1 g/l of Na₂CO₃, temperature of 50° C.) and applying a cathodic current density of 0.8 A/dm² for 2 seconds. After rinsing with de-ionised water, the samples were dipped into a trivalent chromium electrolyte kept at 50° C. composed of: 120 g/l of basic chromium sulphate, 250 g/l of potassium chloride, 15 g/l of potassium bromide and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25° C. by adding sulphuric acid. A Cr—CrOx coating containing between 21-25 mg Cr/m² (measured by XRF) was deposited on the surface by applying a cathodic current density of 10 A/dm² for approximately 1 second, using a platinised titanium anode as counter electrode. The samples so produced showed a shiny metallic appearance.

The study the passivating action of the thin Cr—CrOx coating on tinplate, the samples were subjected to a long-term storage test at 40° C. at a static humidity level of 80% RH. The amount of tin oxide developed on the tinplate surface during storage is then measured after 2 weeks and after 4 weeks of exposure, and compared to the amount of tin oxide present on the sample before the storage test (denoted as ‘0 weeks’). Determination of tin oxide layer thickness is done using a coulometric method, as described in S. C. Britton, “Tin vs corrosion”, ITRI Publication No. 510 (1975), Chapter 4. The tin oxide layer is reduced by a controlled small cathodic current in a 0.1% solution of hydrobromic acid (HBr) that is freed from oxygen by scrubbing with nitrogen. The progress of the reduction of the oxide is followed by potential measurement and the charge passed for the complete reduction (expressed as Coulomb/m² or C/m²) serves as a measure of the tin oxide layer thickness. The results for the sample according to Example 1 are presented in Table 1, including the performance of the reference material, which is the same tinplate material that was passivated using hexavalent chromium, i.e. so-called 311 passivated tinplate.

TABLE 1 Tin oxide layer thickness (in C/m²) Storage at 40° C., ETP - Cr—CrOx according to 80% RH ETP-311 (ref) Example 1 (25 mg/m² Cr) 0 weeks 12 11 2 weeks 12 12 4 weeks 13 11

The results show that non-passivated tinplate treated according to the present invention to obtain a light Cr—CrOx coating shows perfect stability in tin oxide growth and is fully comparable in performance to traditional 311 passivated tinplate.

EXAMPLE 2

Sheets of conventional, non-passivated, flow melted tinplate (common steel grade and temper), with a tin coating weight of 2.8 g Sn/m² on both sides, were first given an electrolytic pre-treatment to minimise the tin oxide layer thickness. This was done by dipping the sheets into a sodium carbonate solution (3.1 g/l of Na₂CO₃, temperature of 50 ° C.) and applying a cathodic current density of 0.8 A/dm² for 2 seconds. After rinsing with de-ionised water, the samples were dipped into a trivalent chromium electrolyte kept at 50° C. composed of: 120 g/l of basic chromium sulphate, 250 g/l of potassium chloride, 15 g/l of potassium bromide and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25 ° C. by adding sulphuric acid. A Cr—CrOx coating containing between 65-75 mg Cr/m² (measured by XRF) was deposited on the surface by applying a cathodic current density of 15 A/dm² for approximately 1 second, using a platinised titanium anode as counter electrode. All samples so produced showed a shiny metallic appearance. A typical SEM image is shown in FIGS. 1 & 2, which shows the deposition of very fine grains of chromium metal—chromium oxide on the tin surface.

The sheets were subsequently lacquered, applying a commercially available epoxy-anhydride lacquer system (Vitalure™ 120 supplied by AkzoNobel). Subsequently, the lacquered sheets were locally deformed by Erichsen cupping.

To analyse the performance of the chromium—chromium oxide coated tinplate several sterilisation tests were done to assess the wet adhesion performance on flat and deformed material. In total 5 different sterilisation media were used during these tests, as shown in Table 2.

TABLE 2 Conditions of sterilisation tests Temperature Time Type Sterilisation medium [° C.] [min] Saline 3.6 wt % NaCl 121 90 Acetic acid 1 wt % CH₃COOH 121 90 Cysteine 3.56 g/l KH₂PO₄ + 121 90 7.22 g/l Na₂HPO₄•2H₂O + 0.5 g/l C₃H₇NO₂S•HCl•H₂O (in buffer solution, pH = 7) Salt-Acid 18.7 g/l NaCl + 121 60 30 g/l CH₃COOH Lactic acid 22.5 g/l C₃H₆O₃ 121 60

After sterilisation the level of lacquer adhesion of the panels was evaluated (by the Cross-cut and tape test (ISO 2409:1992(E)), blister formation (size and number of blisters) and visual discolouration. The overall results are presented in Table 3, including the performance of the reference material, which is the same tinplate material that was passivated using hexavalent chromium, i.e. so-called 311 passivated tinplate. The performance ranking is on a scale from 0 to 5, with 0 being an excellent performance and 5 a very bad performance. The results are averaged over a number of observations, leading to scores with a decimal value.

TABLE 3 Results of lacquer adhesion tests Sterilisation type ETP-311 (ref) ETP - Cr—CrOx Flat Saline 2 1.5 Acetic acid 4 1.5 Cysteine 1 1 Salt-Acid 5 1 Lactic acid 3 2 Dome Saline 2 1 Acetic acid 3.5 1.5 Cysteine 4.5 0.5 Salt-Acid 4 0.5 Lactic acid 3 2.5

The inventors found that the tinplate variant manufactured according to the invention performed consistently equal or better compared to the standard tinplate that is passivated using hexavalent chromium (i.e. the reference).

Striking is the fact that no sulphur staining was found for the material according to the invention, which is difficult to achieve with conventional passivated tinplate and notoriously difficult to achieve with alternative passivations for tinplate that are free of hexavalent chromium.

EXAMPLE 3

A coil of blackplate (common steel grade and temper), not containing any metal coating, was treated in a processing line running at a line speed of 20 m/min. The processing sequence started with alkaline cleaning of the steel by running the strip for approximately 10 seconds through a solution containing 30 ml/l of a commercial cleaner (Percy P3) and 40 g/l of NaOH, which was kept at 60 ° C. During cleaning of the strip an anodic current density of 1.3 A/dm² was applied. After rinsing with de-ionised water, the steel strip was passed through an acid solution for approximately 10 seconds, to activate the surface. The acid solution consisted of 50 g/l H₂SO₄, which was kept at 25° C. After rinsing with de-ionised water, the steel strip was passed into an electroplating tank containing the trivalent chromium based electrolyte kept at 50° C. This electrolyte consisted of: 120g/l of basic chromium sulphate, 250g/l of potassium chloride, 15 g/l of potassium bromide and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25° C. by adding sulphuric acid. The electroplating tank contained a set of anodes consisting of platinised titanium. During processing of the strip a cathodic current density of approximately 17 A/dm² was applied for just over 1 second to electro-deposit a chromium-chromium oxide coating of 60-70 mg Cr/m² (measured by XRF) onto the blackplate surface. All samples so produced showed a shiny metallic appearance. A typical SEM image is shown in FIGS. 1 and 2, which shows the deposition of very fine grains of chromium metal—chromium oxide on the steel surface.

The material so produced, was passed through a coating line to apply a commercially available 20 micrometer thick PET film, through heat sealing. After film lamination, the coated strip was post-heated to temperatures above the melting point of PET, and subsequently quenched in water at room temperature, as per a usual processing method for the PET lamination of metals. The same procedure was followed for the manufacturing of reference material, using a commercially produced coil of ECCS.

The laminated materials were used to produce standard food DRD cans (211×400). In all cases the dry adhesion of the PET film to the can wall was excellent. This was confirmed by measuring the T-peel forces of the PET film on the can wall, which showed similar values for the PET film applied to both the material according to the invention and commercial ECCS (˜7 N/15 mm).

The DRD cans were subsequently filled with different media, closed and exposed to a sterilisation treatment. Some cans were processed that contained a scratch made on the can wall, to simulate and observe the effect of incidental coating damage. An overview of the type of sterilisation tests done is presented in Table 4.

TABLE 4 Conditions of sterilisation tests Temperature Time Type Sterilisation medium [° C.] [min] Saline 3.6 wt % NaCl 121 60 Acetic acid 1 wt % CH₃COOH 121 60 Cysteine 3.56 g/l KH₂PO₄ + 130 60 7.22 g/l Na₂HPO₄•2H₂O + 0.5 g/l C₃H₇NO₂S•HCl•H₂O (in buffer solution, pH = 7)

After the sterilisation treatment the DRD cans were cooled to room temperature, emptied, rinsed and dried for one day. The bottom and can wall were judged visually on the presence of corrosion spots and blisters. The results, as presented in Table 5, show that the sterilisation performance of the material according to the invention is in general somewhat less compared to the ECCS reference. The material seems especially more susceptible to corrosion/coating delamination after coating damage. However, these sterilisation tests are quite severe, so in practice the material according to the invention can be used in specifically selected applications involving sterilisation.

The performance ranking is on a scale from 0 to 5, with 0 being an excellent performance and 5 a very bad performance.

TABLE 5 Results of sterilisation tests Sterilisation type ECCS (ref) BP + Cr—CrOx Saline 1 (1)* 1 (4)* Acetic acid 1 3 Cysteine 0 0 *Symbol in brackets relates to DRD cans with a scratch on the can wall.

EXAMPLE 4

A coil of blackplate (common steel grade and temper), not containing any metal coating, was treated in a processing line identical to that described in the previous example to apply a Cr—CrOx coating.

The sheets cut from this coil were subsequently lacquered, applying a commercially available epoxy-phenol lacquer system (Vitalure™ 345 supplied by AkzoNobel). Subsequently, the lacquered sheets were locally deformed by Erichsen cupping.

To analyse the performance of the chromium—chromium oxide coated blackplate several sterilisation tests were done to assess the wet adhesion performance on flat and deformed material. In total 5 different sterilisation media were used during these tests, as shown in Table 6.

TABLE 6 Conditions of sterilisation tests Temperature Time Type Sterilisation medium [° C.] [min] Saline 3.6 wt % NaCl 121 60 Acetic acid 1 wt % CH₃COOH 121 60 Cysteine 3.56 g/l KH₂PO₄ + 130 60 7.22 g/l Na₂HPO₄•2H₂O + 0.5 g/l C₃H₇NO₂S•HCl•H₂O (in buffer solution, pH = 7) Salt-Acid 18.7 g/l NaCl + 121 60 30 g/l CH₃COOH Lactic acid 22.5 g/l C₃H₆O₃ 100 30

After sterilisation the panels were evaluated with respect to the level of lacquer adhesion (by the Cross-cut and tape test (ISO 2409:1992(E))), blister formation (size and number of blisters) and visual discolouration. The overall results are presented in Table 7, including the performance of the reference material, for which commercially available ECCS was used. The performance ranking is on a scale from 0 to 5, with 0 being an excellent performance and 5 a very bad performance.

TABLE 7 Results of sterilisation tests Sterilisation type ECCS (ref) BP + Cr—CrOx Flat Saline 0 0 Acetic acid 0 0 Cysteine 0 0 Salt-Acid 0 0 Lactic acid 0 0 Dome Saline 0 0 Acetic acid 5 4 Cysteine 0 0 Salt-Acid 0 0 Lactic acid 0 0

The inventors found that the Cr—CrOx coated blackplate material manufactured according to the invention performed consistently similar to conventional ECCS.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 show typical SEM images, which show the deposition of very fine grains of chromium metal-chromium oxide onto the surface. FIG. 1 relates to a tinplate substrate and FIG. 2 relates to a blackplate substrate.

FIG. 3 shows an overview of various packaging applications. On the X-axis are packaging steel grades, and on the Y-axis a typical thickness range is shown for these applications for which the packaging steel substrate according to the invention could be used.

FIG. 4 shows where the current is plotted against the anode potential in SHE and FIG. 5 shows a schematic drawing of a gas diffusion anode. 

1. A process for producing a coated steel substrate for packaging applications by depositing a chromium metal—chromium oxide coating on the substrate for packaging applications containing a) a conventional non-passivated electrolytic, optionally flowmelted, tinplate, Or b) a cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate, comprising electrolytically depositing on said substrate said chromium metal—chromium oxide coating in a single process step from a plating solution comprising a mixture of a trivalent chromium compound, a chelating agent, an optional conductivity enhancing salt, an optional depolarizer, an optional surfactant, to which an acid or base is optionally added to adjust pH, wherein the plating solution does not contain a buffering agent, and wherein a sufficiently high cathodic current density is being applied to deposit chromium metal.
 2. The process according to claim 1, wherein the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation and the depolarizer comprises a bromide containing salt.
 3. The process according to claim 1, wherein the chelating agent, the conductivity enhancing salt and the depolarizer is potassium contains a cationic species and the cationic species in the chelating agent, the conductivity enhancing salt and the depolarizer is potassium.
 4. The process according to claim 1, wherein the coated substrate is further provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a lacquering step, or a thermoplastic single layer, or a thermoplastic multi-layer polymer by a film lamination step or a direct extrusion step.
 5. The process according to claim 1, wherein an anode is chosen that reduces or eliminates oxidation of Cr(III) ions to Cr(VI) ions during the plating step.
 6. The process according to claim 1, wherein the substrate is a tin coated substrate for packaging applications and is subjected to an electrolytic pre-treatment to minimise tin oxide layer thickness before coating one or both sides with the chromium metal—chromium oxide coating layer.
 7. The process according to claim 6, wherein the electrolytic pre-treatment consists of dipping the tin coated substrate into a sodium carbonate solution and applying a cathodic current density.
 8. The process according to claim 7, wherein the sodium carbonate solution has 2 to 5 g/l of Na₂CO₃ at a temperature of between 35 and 65° C., and wherein the cathodic current density of between 0.5 and 2 A/dm² is applied for a period of between 0.5 and 5 seconds.
 9. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at least 20 mg/m².
 10. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at most 140 mg/m².
 11. The process according to claim 1, wherein the electrolytic deposition depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 20 mg/m².
 12. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 140 mg/m².
 13. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium meta—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at least 40 mg/m².
 14. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the non-passivated tinplate containing a total chromium content of at most 90 mg/m².
 15. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 40 mg/m².
 16. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at least 60 mg/m².
 17. The process according to claim 1, wherein the electrolytic deposition deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 90 mg/m².
 18. The process according to claim 1, wherein the electrolytic depositing of the chromium metal-chromium oxide layer deposits a chromium metal—chromium oxide layer on the cold-rolled and recovery annealed electrolytic, optionally flowmelted, tinplate containing a total chromium content of at most 80 mg/m².
 19. The process according to claim 4, wherein the thermoplastic single layer, or a thermoplastic multi-layer polymer is a polymer coating system comprising one or more layers comprising a thermoplastic resin selected from the group consisting of polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers; and/or copolymers thereof; and/or blends thereof.
 20. The process according to claim 1, wherein an anode is a gas diffusion anode that reduces or eliminates the oxidation of Cr(III) ions to Cr(VI) ions during the plating step. 